1. Field of the Invention:
This invention relates to a fractional-division frequency synthesizer for obtaining an angle-modulation signal.
2. Description of the Prior Art:
A frequency synthesizer of the fractional-division type is already well-known and has the intended function of delivering an output signal of the form F.sub.S =(N+k)F.sub.R where N is the integral part of a number N+k and F.sub.R is the frequency of a reference signal. This synthesizer comprises a variable oscillator and an oscillator control loop which in turn comprises in series a variable divider having a preselected divisor, a comparator for comparing the phase of the output signal of the divider with the phase of the reference signal, a summing device and a filter. The synthesizer further comprises a phase accumulator which performs modulo-M summation of the number G=k(M) at the frequency F.sub.R, the sum and carry outputs of said accumulator being coupled respectively to the summing device in order to deliver a compensation signal and to the divider in order to deliver a control signal to select the divisor N+1.
When a frequency synthesizer of this type is intended to be associated with a digital-signal modulator for delivering a signal which is angle-modulated by digital data, it is a known practice to place the modulator in the control loop or at the output of the frequency synthesizer. When the modulator is placed in the control loop between the summing device and the control input of the variable oscillator, the control loop behaves as a high-pass filter for the modulation signals which are applied to the control input of the variable oscillator and there is no possibility of transmitting the low-frequency components of the digital information. Another way of eliminating the loop passband problem consists in providing a fixed-frequency angle modulator by means of a voltage-controlled quartz oscillator, the signal delivered by the latter being added to the signal of the frequency synthesizer by means of mixing followed by filtering in order to remove the undesirable mixing products. This method is costly, however, one of the main reasons being that it is solely of the analog type. It proves necessary in each case to place an analog filter at the input of the modulator in order to limit the spectrum of the modulated signal.